Differential capacitive sensing circuit for a multi-electrode capacitive force sensor

• A new approach for differential capacitive sensing using a relaxation oscillator.
• Extensive noise analysis to reveal the influence of each component on the measurement noise.
• Accuracy of up to 0.12 fF for a capacitance of 5.5 pF, which can be improved further by optimizing component values.
• Successfully interfaced with a MEMS force sensor to prove its functionality.
• Sensing principle can be used in many other (differential) capacitive sensor applications.

A multi-electrode differential capacitive sensing circuit is designed and realized for the read-out of a multi-axis capacitive force–torque sensor. The sensing circuit is based on a differential relaxation oscillator, to which multiple capacitances can be connected. For selecting the capacitances, reprogrammable asynchronous logic can be used, such that any desired combination of differential or single-ended capacitance can be determined. The noise performance of the oscillator in the system is analysed and measured, revealing the influence of individual component values on the noise performance of the system. Capacitance measurements show that a deviation of 0.9 fF is obtained at an acquisition rate of 225 Hz including auto-calibration, which is mainly limited by the quantization noise due to the frequency counter. The lowest obtained deviation is 0.12 fF at an acquisition rate of 3.5 Hz. The system is successfully interfaced to the multi-axis capacitive force–torque for the read-out of six capacitor configurations at an acquisition rate of 38 Hz.